[varmintman]

Recent models show that physical limitations on their size was neither bone nor muscle strength, but joints.

A chain is no stronger than its weakest link. The fact that the limitation imposed by muscle dynamics is approximately 20,000 lbs means that any larger limit allowed by consideration of joints or anything else is irrelevant.

[FredZarguna]

And yet, we have elephants as recently as 10K-y ago, which weighed over 40,000 lbs. Was the Earth’s gravity also much weaker as recently as a mere 100 centuries ago in your “theory?”

[BroJoeK]

varmintman: "The fact that the limitation imposed by muscle dynamics is approximately 20,000 lbs means that any larger limit allowed by consideration of joints or anything else is irrelevant. "

But that is not a "fact", and you should be ashamed of yourself for spreading cockamamie lies.
The real fact of this matter is that other, more recent mammal bones have been found of land-critters ranging up to twice the size of your imaginary 20,000 lbs. limit.

Those fossils include Indricotherium from 25 million years ago and Songhua River Mammoth from as recently as 10,000 years ago.

So, there is no evidence -- zero, zip, nada -- supporting your fantasies about changing gravitation.